The invention relates to a switching arrangement for controlling the speed of hydraulic drives.
The invention relates to hydraulic drives in which the speed derived from the drives depends on the amount of hydraulic medium directed to the drive within a given time unit, wherein the absorption amount of the drive generally is not variable. This invention is applicable to various known hydraulic drives which, as a rule, are constructed in a simple manner. In practice, such drives serve various purposes, such as the supply of kinetic energy to slewing gears of steam shovels and cranes. The invention relates in particular to so-called thrust pistons, which produce a kinetic energy via a reversible piston of a hydraulic working cylinder. For this reason, the invention is described in the following text on the basis of this preferred application.
Generally, hydraulic thrust pistons are driven at a constant speed from a standstill point and are fully braked from the respective speed. In these cases, a 4/3 distributing control valve slide is used which, in both extreme positions, alternates the pressure and the tank line. However, in the middle position, the 4/3 distributing control valve slide switches the pump contained in the thrust generator to circulation, i.e. to the tank of the hydraulic medium. With such drives, the piston speeds are nearly constant in both directions. If the thrust piston is driven by the load in one or both directions of movement, a load holding valve is required on the outlet side, i.e. a hydraulic pilot controlled check valve or brake valve is required to prevent the thrust piston from passing and to let the piston run in the cylinder at a speed corresponding to the predetermined flow of the hydraulic medium.
Such a hydraulic switching arrangement, also described as an on-off control, is provided, for example, for the drive cylinders of pivotable booms or masts. For example, crane of concrete distributor masts have hydraulically driven joints that connect the mast section(s) to each other. Here, the drive cylinders are regularly under the load of the mast section extending from the respective joint and possibly under the loads effecting same, such as a concrete supply line. The on-off control has the property of fully applying pressure to the drives. This results in jerky starts and braking of the joint movement. Apart from the wear connected therewith, abrupt starting and braking frequently causes the mast to oscillate, thus, possibly increasing the mechanical load on the structural components to a dangerous level, possibly resulting in accidents and damage.
Attempts have already been made to counteract such disadvantages. One of these suggestions involves supplementing the described on-off switching arrangement with a throttle or restrictor having a constant cross-section (throttle), whereby the disadvantageous oscillating jolts are eliminated. Such switching arrangements also make it possible with a thrust generator, having a pump with constant supply flow, to supply several hydraulic circuits with respectively varying partial amounts of hydraulic medium which are independent of the respective operating pressures present in the circuits. Furthermore, in the inlet and outlet between the control valve and the working cylinder, respectively, a throttle is built in, wherein the throttles are so attuned to each other that the throttle in the outlet, during maximum load including the blocking pressure load, lets through the amount of hydraulic medium destined for this circuit; in contrast thereto, the nozzle in the inlet is adjusted in such a way that when the amount of hydraulics medium destined for this circuit is let through, there exists behind the throttle the required unlocking pressure, while in front of the throttle, the relief valve pressure is barely reached.
While with such a switching arrangement merely the oscillation jolts are reduced, it is possible with proportional controls to achieve continuous speed alternations of the speed derived from the drive which constitutes a substantially better means for reducing the starting and braking jolts. With a proportional control of this type, a control valve is provided in front of the drive which electromagnetically alters the opening cross-section of fine control grooves which is nearly proportional to the speed. The electromagnetic adjustment of the control valve and thus that of its pilot control grooves involves a potentiometer circuit which further considerably increases the already elevated expense, due to the electrical components. Furthermore, such switching arrangements, due to the fine control grooves, are susceptible to soiling so that frequent disruptions are unavoidable, which are mostly brought about due to jamming of the valve. Moreover, the handling of such controls via the command switch is difficult because the latter has to be very delicately adjusted.